Method for producing an improved liquid cystal device, and resulting device

ABSTRACT

The present invention relates to a method of making nematic liquid crystal devices, characterized by the fact that low energy zenith anchoring of the liquid crystal is obtained over at least one of the confinement plates by the steps consisting in:  
     depositing a polymer or copolymer based on PVC onto a substrate;  
     stabilizing the polymer deposit; and  
     defining an azimuth orientation for the deposit to induce controlled azimuth anchoring of the liquid crystal.

[0001] The present invention relates to the field of liquid crystal display devices.

[0002] More precisely, a main object of the present invention is to propose novel means for anchoring liquid crystal molecules in display cells.

[0003] In general, liquid crystal display devices comprise two confinement plates or substrates having electrodes on their inside surfaces and a liquid crystal material placed between the two plates. Means are also provided on the confinement plates for anchoring and controlling the anchoring of liquid crystal molecules.

[0004] The person skilled in the art knows in particular how to orient nematic liquid crystals either perpendicularly to the surface of the substrate (this orientation is said to be “homeotropic”) or else parallel thereto (this orientation is said to be “planar”), using surface treatments such as depositing polymers.

[0005] Still more precisely, “helical nematic” liquid crystal displays require anchoring that is strong and planar, or more precisely slightly oblique. Such strong planar anchoring is usually obtained in the prior art by depositing orientated polymers or by evaporating SiO at a given angle.

[0006] Another family of liquid crystal displays has recently appeared (document WO-97/17632), in which family the nematic displays operate by breaking surface anchoring. Those displays are bistable and the display therefore remains indefinitely in a state selected from two possible states without consuming energy; energy is required only to switch between two states. Another advantage of being bistable is the possibility of achieving high definition since the number of display lines is a function only of the ratio of the time available before the information needs to be changed and the time required for writing one line. This makes it possible to have more than 10,000 lines. The major difficulty in making displays of that type is nevertheless associated with the need to break the anchoring on the surface: it is necessary for the anchoring to be reproducible and to require little energy. The main utility of the present invention lies in making displays of that type.

[0007] A typical example of such bistable nematic displays using weak anchoring is made as follows. The liquid crystal is placed between two plates of glass coated in a conductive layer of indium and tin oxide (ITO). One of the electrodes carries a coating that gives strong planar oblique anchoring, and the other electrode carries a coating that gives planar monostable anchoring with weak anchoring energy in the zenith direction and medium or strong anchoring energy in the azimuth direction. Two polarizers are also placed on either side of the cell with appropriate orientations.

[0008] The principle of this “bistable” technology lies in the presence of two stable states without any need to apply an electric field, a uniform state, and a state twisted by 180°. Those two states correspond to minimum energy levels. They are balanced by using a nematic liquid crystal with positive dielectric anisotropy, for example pentyl cyano biphenyl (known as “5CB”) doped with a chiral additive.

[0009] The display technology makes use of the possibility of breaking the weak planar anchoring and makes use of the persistence of the strong planar anchoring to switch from one state to the other, by applying an electric field of specific shape and intensity. This field extends perpendicularly to the cell and induces a homeotropic texture that is close to the “black” state in twisted nematic (TN) technology but in which the molecules close to the low anchoring energy surface are perpendicular thereto. This non-equilibrium texture is the transition state that makes it possible to switch to one or other of the two stable states. When the field is removed, the state will convert to one or other of the stable states depending on which elastic or hydrodynamic coupling effects have been favored.

[0010] Various means have already been proposed for anchoring liquid crystal materials.

[0011] Nevertheless, it turns out to be very difficult to identify means that give full satisfaction because of the numerous constraints with which the anchoring means must comply.

[0012] In particular, the person skilled in the art knows that the anchoring means must: 1) be soluble in a medium in order to be applied appropriately to the receiving plate or substrate so as to wet it and cover it properly; 2) be suitable for making the plate or substrate aniostropic; 3) be non-soluble in the liquid crystal material; and 4) define anchoring that gives the desired orientation and energy.

[0013] Although means are already available that are suitable for defining satisfactory strong anchoring, there are very few means available that are suitable for providing weak anchoring (this concept is explained in greater detail below).

[0014] An object of the present invention is thus to propose novel means suitable for defining weak anchoring.

[0015] In the present invention, this object is achieved by a method of preparing liquid crystal cells, the method comprising the steps consisting in:

[0016] depositing a polymer or copolymer based on polyvinylchloride (PVC) onto a substrate;

[0017] stabilizing the polymer deposit; and

[0018] defining an azimuth orientation for the deposit to induce controlled azimuth anchoring of the liquid crystal.

[0019] According to an advantageous characteristic of the present invention, the polymer deposit is stabilized thermally and/or by ultraviolet exposure.

[0020] The present invention makes it possible to provide low energy anchoring layers (alignment layers) in liquid crystal display cells, and in particular bistable nematic liquid crystal cells.

[0021] According to other advantageous characteristics of the present invention:

[0022] the stabilization implements exposure to ultraviolet light;

[0023] the stabilization implements one or more annealing steps;

[0024] the stabilization uses one or more annealing steps performed before and/or after exposure;

[0025] the polymer is selected from the group comprising PVC and a copolymer of PVC and polyvinylacetate (PVAc);

[0026] the polymer is a terpolymer based on PVC-PVAc;

[0027] the polymer is a terpolymer based on PVC-PVAc in which the third monomer is hydroxypropyl acrylate or vinyl alcohol;

[0028] the heating step(s) is/are performed at temperatures below the melting point (mp) of the polymer;

[0029] the heating step(s) is/are performed for a duration of about one hour and thirty minutes (1 h 30);

[0030] the exposure step is performed using ultraviolet light having a wavelength lying in the range 180 nanometers (nm) to 400 nm;

[0031] the step consisting in defining controlled azimuth anchoring of the liquid crystal is performed by the fact that the polymer of the invention (PVC and PVC-PVAc copolymers) is rubbed with a textile-covered roller;

[0032] the step consisting in defining controlled azimuth anchoring of the liquid crystal is performed by depositing the polymer on a substrate that has already been made anisotropic by special treatment such as: another brushed or stretched polymer; evaporated SiO or other oxide; a grating (etched, printed, or photoinduced);

[0033] the polymer is deposited on the substrate by centrifuging;

[0034] the polymer is deposited in solution in a solvent based on ketone, such as methyl ethyl ketone or dimethyl formamide; and

[0035] the anchoring layer having low zenith energy is deposited on an electrode that is transparent or reflecting.

[0036] The present invention also provides devices obtained in this way.

[0037] The notions of “strong” anchoring and “weak” anchoring are explained below.

[0038] The terms “strong anchoring” and “weak anchoring” can be defined as explained below. The origin of the molecular anchoring of the liquid crystal is anisotropy in its interactions with the orientation layer. Molecular anchoring can be characterized by its effectiveness and by the direction it imparts to the liquid crystal molecules in the absence of any other external influence. This direction is known as the “easy” axis and it is represented by the unit vector {right arrow over (n)}_(o) or by its zenith and azimuth angles θ_(o) and φ_(o) in the Cartesian coordinate system having its Z axis perpendicular to the surface of the substrate.

[0039] If the easy axis of the liquid crystal molecules is perpendicular to the substrate, then the alignment is “homeotropic”. If it is parallel to the substrate, then the alignment is “planar”. Between those two extremes, alignment is said to be “oblique” and is defined by the zenith anchoring angle given relative to the normal to the surface of substrate, or by its complement preferred to as the “pretilt” angle.

[0040] The strength of the interaction between the liquid crystal and the substrate is characterized by reference to energy density per unit area γ. It depends on the orientation {right arrow over (n)}_(s) of the liquid crystal molecules on the surface (also defined by the zenith and azimuth angles θ_(s) and φ_(s)):

γ(θ_(s), φ_(s))=γ(θ_(o), φ_(o))+g(θ_(s)−θ_(s), φ_(s)−φ_(o))

[0041] where g represents the anchoring energy. It characterizes the anisotropic portion of the interaction and, by convention, it becomes zero when the molecular orientation of the liquid crystal coincides with the direction of the easy access.

[0042] In most experiments, variation in one or other of the two angles (zenith or azimuth) is preponderant. That is why the two components of anchoring energy are often studied separately. The best known form anchoring energy is that proposed by Rapini and Papoular (PG de Gennes “J. Phys. Coll.”, 30, C-4, (1969)): ${g\left( {\theta,\varphi} \right)} = {{\frac{w_{z}}{2}{\sin^{2}(\theta)}} + {\frac{w_{a}}{2}{\sin^{2}(\varphi)}}}$

[0043] The positive coefficients w_(z) and w_(a) are usually referred respectively as the zenith anchoring energy and as the azimuth anchoring energy. They have the dimensions of energy density per unit area.

[0044] Anchoring energy can also be given by the extrapolation length. This is the distance between the surface under study and the position of a virtual surface. By imposing infinitely strong anchoring (where it is impossible to cause the molecules situated on the virtual surface to pivot), it induces the real texture of the liquid crystal. The zenith extrapolation length L_(z) is proportional to the inverse of the anchoring energy w_(z) i.e. L_(z)=k₁₁/w_(z) where k11 is the elasticity coefficient for fan deformation of the liquid crystal in question. Similarly, the azimuth extrapolation length is defined as L_(a)=k₂₂/w_(a), where k₂₂ is the elasticity coefficient for twisting deformation of the liquid crystal in question. As a general rule, anchoring is said to be “strong” when the molecules at the surface remain practically parallel to the easy access while the cell is in operation. In contrast, anchoring is said to be “weak” when if significant deflection appears in operation.

[0045] Zenith anchoring energy can be evaluated by a simple method described below: measuring the crystal field for breaking anchoring.

[0046] It is known that in a liquid crystal cell, surface anchoring can be “broken” by using an electric field E>E_(c) normal to the plates, applied to a nematic liquid crystal having positive dielectric anisotropy ε_(a)=ε_(//)−ε_(⊥)>0. For E increasing and approaching E_(c), the surface molecular angle θ_(s) passes quickly from 90° to 0. This corresponds to detectable variation in the birefringence of the cell. Above E_(c), the angle θ_(s) remains zero, and the surface is said to be “broken”.

[0047] The critical field E_(c) for breaking the anchoring is defined by the following condition: $E_{c} = {\frac{1}{L_{z}}\sqrt{\frac{K}{ɛ_{o}ɛ_{a}}}}$

[0048] K is the elastic constant of curvature (≈10 pico-newtons (pN)) and L_(z) is the extrapolation length defining the zenith anchoring energy which is written as follows:

w _(z)=(½)(K/L _(z))cos²θ_(s) (where θ_(s) is the angle of the surface molecules).

[0049] For zenith anchoring, it is considered that the anchoring is strong if L_(z)<20 nanometers (nm) (E_(c)>20 volts per micrometer (V/μm)) and weak if L_(z)>40 nm (E_(c)<10 V/μm). Azimuth anchoring is an order of magnitude smaller. Azimuth anchoring is considered to be strong is L_(a)<100 nm.

[0050] In the context of the present invention, low energy zenith anchoring is obtained by depositing specially selected polymers based on PVC subjected to a sequence of specific treatments.

[0051] The anchoring layer based on PVC is deposited from a polymer solution by centrifuging. After the solvent has evaporated, a layer of polymer is obtained having a thickness that typically lies in the range a few nanometers to 100 nm (although this is not limiting on the invention).

[0052] Thereafter, it is preferable to apply rubbing using a textile roller of the prior art to the polymer layer in order to impose an azimuth orientation thereon so as to induce azimuth anchoring of the liquid crystal.

[0053] To make a cell that is bistable by breaking anchoring, the “weak” anchoring surface must present planar anchoring with relatively weak zenith anchoring energy (e.g. L_(z)≧40 nm) and azimuth anchoring that is relatively strong (L_(a)<<d, where d is the thickness of the cell).

[0054] The polymers and copolymers of the invention are PVC (I) and PVC-PVAc copolymers (II) having the following formulas:

[0055] By way of example PVC-PVAc copolymers (II) of the invention have 80:20 compositions and can vary.

[0056] Terpolymers derived from PVC, PVAc and a co-monomer also enable low energy anchoring layers to be obtained and constitute a variant of the invention. An example of a suitable copolymer is hydroxypropyl acrylate or vinyl alcohol.

[0057] The polymers and copolymers are deposited on the substrate by centrifuging a solution. Appropriate solvents are constituted, for example, by ketones such as methyl ethyl ketone or dimethyl formamide.

[0058] After annealing at a temperature below the melting point (mp), (and higher than the vitreous transition temperature Tg), preferably between 140° C. and 150° C., for a length of time lying in the range a few minutes to a few hours (typically 30 min to 2 h, e.g. about 1 h 30), the polymer layer is exposed to UV radiation having a wavelength lying in the range 180 nm to 400 nm, and optionally annealed at a temperature lying in the range 130° C. to 170° C. The polymer layer is then rubbed using a roller in order to induce azimuth orientation.

[0059] The layers are generally made on a conventional ITO substrate, but other electrodes could be envisaged.

[0060] In a variant of the invention, the azimuth orientation of the anchoring layer can be obtained by methods other than rubbing with a roller, for example by using a substrate that has previously been treated:

[0061] by oblique evaporation of SiO;

[0062] by a polymer brushed with a roller or stretched; or

[0063] by a grating etched in the substrate, printed, or photoinduced.

[0064] The method of the present invention makes it possible to obtain an alignment layer with weak zenith anchoring.

[0065] Two example implementations of the invention are described below.

[0066] A first example enables a liquid crystal cell having a thickness of 1.3 microns to be assembled between two ITO covered glass plates. One plate received an evaporation of SiO (thickness 107 nm, angle of incidence 82.50) in order to obtain highly inclined anchoring. The other electrode was covered in PVC to a thickness of 20 nm to 30 nm and treated using the method of the invention:

[0067] Step 1: the polymer was deposited by centrifuging from a 0.5% by weight solution in methyl ethyl ketone;

[0068] Step 2: annealing at 150° C. for 1 h 30;

[0069] Step 3: exposure (λ=254 nm) for 20 hours to a 6 watt mercury vapor lamp;

[0070] Step 4: annealing at 150° C. for 30 minutes; and

[0071] Step 5: brushing to induce azimuth anchoring with a roller covered in a textile velvet.

[0072] The two above-described plates were assembled together to make a cell in compliance with the concept described in document WO 97/17632. The cell was filled with doped 5 CB liquid crystal (chiral pitch=5.6 microns). The cell was operated using 13 volt pulses at ambient temperature and presented a contrast ratio of 50.

[0073] In another example of the invention, the liquid crystal used was a mixture adapted from the technology complying with the concept described in document WO 97/17632. The operating range then extended from 18° C. to more than 60° C. using the same 13 volt control pulses.

[0074] The weak planar anchoring prepared by the invention has the following characteristics:

[0075] the alignment layers are chemically and mechanically stable over time, and in contact with the liquid crystal mixtures used;

[0076] the anchoring is planar, the pretilt is zero, to within the accuracy of the measurements used, i.e. less than 0.2°; and

[0077] the extrapolation length of the zenith anchoring as measured by a strong field technique for 5 CB at a temperature of 22° C. is close to L_(z)=50 nm and varies little as a function of the polymer used in accordance with the invention (PVC-PVAc).

[0078] Several commercially available nematic mixtures were tested and the extrapolation length lay in the range 30 nm to 60 nm as a function of the mixture and the temperature. These values correspond to zenith anchoring energies that are much lower than those obtained with the conventional polymers used in helical nematic displays.

[0079] The azimuth anchoring force obtained depends on the treatment performed on the layers. By way of example, with brushing azimuth extrapolation lengths were obtained lying in the range 50 nm to 200 nm. These values are compatible with cells operating in compliance with the concept described in document WO 97/17632.

[0080] The present invention presents the following advantages in particular. In the state of the art, making low energy anchoring, i.e. using a layer of silicon oxide (SiO) needs to be performed in a vacuum. The vacuum deposition process is length and expensive and it is difficult to monitor.

[0081] Compared with the known process for making low energy anchoring, using a polymer layer of the present invention offers significant advantages in terms of simplicity and reduced manufacturing costs.

[0082] Naturally, the present invention is not limited to the particular embodiments described above, but it extends to any variant within the spirit of the invention. 

1/ A method of making nematic liquid crystal devices, the method being characterized by the fact that it comprises the steps consisting in: depositing a polymer or copolymer based on PVC onto a substrate; stabilizing the polymer deposit; and defining an azimuth orientation for the deposit to induce controlled azimuth anchoring of the liquid crystal; in order to obtain low energy zenith anchoring for the liquid crystal over at least one of its confinement plates. 2/ A method according to claim 1, characterized in that the stabilization implements exposure to ultraviolet light. 3/ A method according to claim 1 or claim 2, characterized in that the stabilization implements one or more annealing steps. 4/ A method according to any one of claims 1 to 3, characterized in that the stabilization uses one or more annealing steps performed before and/or after exposure. 5/ A method according to any one of claims 1, 2, 3, or 4, characterized by the fact that the polymer is selected from the group comprising PVC and a copolymer of PVC and PVAc. 6/ A method according to any one of claims 1, 2, 3, or 4, characterized by the fact that the polymer is a terpolymer based on PVC-PVAc. 7/ A method according to claim 6, characterized by the fact that the polymer is a terpolymer based on PVC-PVAc in which the third monomer is hydroxypropyl acrylate or vinyl alcohol. 8/ A method according to any one of claims 1 to 7 in combination with claim 3 or claim 4, characterized by the fact that the heating step(s) is/are performed at temperatures below the melting point (mp) of the polymer. 9/ A method according to any one of claims 1 to 8 taken in combination with claim 3 or claim 4, characterized by the fact that the heating step(s) is/are performed for a duration of about 1 h
 30. 10/ A method according to any one of claims 1 to 9 taken in combination with claim 2, characterized by the fact that the exposure step is performed using ultraviolet light having a wavelength lying in the range 180 nm to 400 nm. 11/ A method according to any one of claims 1 to 10, characterized by the fact that the step consisting in defining controlled azimuth anchoring of the liquid crystal is performed by the fact that the polymer of the invention (PVC and PVC-PVAc copolymers) is rubbed with a textile-covered roller. 12/ A method according to any one of claims 1 to 10, characterized by the fact that the step consisting in defining controlled azimuth anchoring of the liquid crystal is performed by depositing the polymer on a substrate that has already been made anisotropic by special treatment such as: another brushed or stretched polymer; evaporated SiO or other oxide; a grating (etched, printed, or photoinduced). 13/ A method according to any one of claims 1 to 12, characterized by the fact that the polymer is deposited on the substrate by centrifuging. 14/ A method according to any one of claims 1 to 13, characterized by the fact that the polymer is deposited in solution in a solvent based on ketone, such as methyl ethyl ketone or dimethyl formamide. 15/ A bistable nematic liquid crystal device using at least one anchoring layer with low zenith energy made using the method according to any one of claims 1 to
 14. 16/ A device according to claim 15, characterized by the fact that the polymer is selected from the group comprising: PVC, a copolymer of PVC and PVAc, a terpolymer based on PVC and PVAc. 17/ A device according to claim 15 or claim 16, characterized by the fact that the anchoring layer having low zenith energy is deposited on an electrode that is transparent or reflecting. 